A linear actuator converts a given input energy, such as electric, pneumatic, or hydraulic energy, into mechanical motion along a straight line. Conventionally, linear actuators have been based on magnetic actuators like solenoids or have used a mechanism to convert rotary motion of a DC motor into linear motion like the lead screw mechanism or a cam arrangement. With the advancement in piezoelectric devices, some linear actuators have been designed using piezoelectric materials. The stack actuator is the simplest and the most common piezoelectric linear actuator which consists of multiple piezoelectric discs stacked together. The displacement of each layer of discs adds up resulting in the final displacement. However, in spite of having multiple layers, the maximum displacement of a typical stack actuator is limited to a few microns which is substantially low for most of the practical applications.
Some researchers have added amplification mechanisms to a stack actuator to increase the stroke length. For example, some piezoelectric linear actuators employ bridge type flexure mechanism for displacement amplification of stack actuators. However, the corresponding drawback of such bridge type stroke amplification mechanisms is that the force output is reduced substantially as a result. In other examples, some piezoelectric linear actuators employ hydraulic amplification mechanisms for displacement amplification; however, one of the primary drawbacks of hydraulic amplification mechanisms is that they are prone to leakage. Moreover, having mechanical components like moving pistons, valves and pumps further intensifies reliability concerns.
Furthermore, most of the linear actuators encountered in commercial or industrial use utilize friction to achieve linear motion therein. For example, a rotating DC motor with a lead screw mechanism relies on friction between the threads of the lead screw and the nut or the bearings to transform the rotary motion to a linear motion. Even most of the piezoelectric linear actuators such as Piezo-Walk® are progressive motion devices which rely on friction between a guide and actuating elements to push the guide forward in successive steps. Because of friction between its components, such linear actuators may have high wear and tear, and may even lead to failure of the device.
From the above discussion it may be understood that there are a few shortcomings of existing linear actuators and piezoelectric actuators in particular. Since the linear motion is achieved through a series of steps, their speeds are limited, especially for fast oscillatory motion. Existing piezoelectric linear actuators need custom electronic circuits with multiple drive signals since the motion of the various elements have to be coordinated sequentially to produce the desired linear motion. Further, since they are friction based, they are expected to have high wear and tear, and large amount of energy dissipation as heat.
In light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks of conventional linear actuators and develop a linear actuator capable of providing oscillatory motion up to a few kHz with displacement in the millimeter range along with the capability of scaling the force output to a desired level.